Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials. The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer).
A transparent conductive oxide (“TCO”) layer is commonly used between the window glass and the junction forming layers. For example, the TCO layer may be sputtered from a cadmium stannate (i.e., Cd2SnO4) target by either of two processes: hot sputtering or cold sputtering. When hot sputtered, the TCO layer is typically deposited at sputtering temperatures above about 250° C. in a one step sputtering process. When cold sputtered (e.g., at about room temperature), the TCO layer must be annealed following sputtering of the layer in a second step to convert the layer from an amorphous layer to a crystalline layer.
Though the hot sputtering process is more streamlined (i.e., only requiring a single step), the hot sputtered TCO layers can have a much higher resistivity than the cold sputtered TCO layers—even when sputtered from the same material (e.g., cadmium stannate)—making the hot sputtered TCO layer less attractive for the end use. Although not wishing to be bound by any particular theory, it is believed that this difference in resistivities between the hot sputtered layer and the cold sputtered layer likely stems from a difference in the as-deposited stoichiometry. For example, when sputtering from a cadmium stannate target, it is presently believed that cold sputtering produces a layer having the stoichiometry Cd2SnO4, which is the desired stoichiometry for cadmium stannate. However, other processing issues exist that hinders the viability of cold sputtering to form the TCO layer, especially from a cadmium stannate target. For example, the annealing process can sublimate cadmium atoms off of the TCO layer, altering the stoichiometry of the TCO layer, especially along its outer surface.
To counteract this loss of cadmium atoms from the surface of the TCO layer, the TCO layer is typically annealed in contact with an anneal plate containing cadmium. For instance, an anneal plate having cadmium sulfide on its surface contacting the TCO layer can be used to provide additional cadmium to the TCO layer during annealing to inhibit the loss of cadmium from the TCO layer.
However, contact plate is awkward for manufacturing use in a large commercial-scale manufacturing setting, and becomes depleted of cadmium during repeated use, requiring plate change. As such, the use of such anneal plates adds manufacturing processes and materials, effectively increasing the manufacturing cost and complexity for formation of the PV modules. Thus, a need exists for methods of forming a TCO layer having the conductivity of cold sputtered layers with the processing ease found in those hot sputtered layers.